In brushless motor systems the motor rotor position must be sensed for commutation and position/speed control. Traditionally this has been accomplished with a combination of digital Hall sensors & incremental encoders, absolute encoders or resolvers.
Historically in brushless motor systems, commutation sensors have been used for sensing the relative position of the rotor to the stator allowing the control to generate the magnetic fields required to produce motion of the rotor. Additionally servo systems have used a high resolution feedback to supply real time position or speed information to the controller. High resolution feedback produces a constant supply of position information allowing the control to make continuous adjustments to speed, thus maintaining the desired motion/position.
Incremental encoder systems are required to provide two separate signal types to the controller, commutation signals and high resolution signals. This can be accomplished with separate Hall Effect digital switches for the commutation signals with the encoder providing the high resolution signals. Some encoders provide separate commutation outputs built into the encoder eliminating the need for separate Hall Effect switches as shown in FIG. 1. The commutation outputs are absolute over 1 electrical cycle of the motor (referred to as U,V and W in FIG. 1) these sense the mechanical alignment of the motor's rotor relative to the stator. The encoder incremental outputs (referred to as A and B in FIG. 1) are 90 degrees out phase from each other and provide rotation direction and high resolution feedback used by the control for position and speed control.
A resolver, on the other hand, is a multi-winding transformer in which the output ratio varies with rotor position. A typical resolver is illustrated in FIG. 2. Typically, this system requires a reference frequency, shown in FIG. 2 as the oscillator, and requires a resolver to digital converter integrated circuit for interface to a processor.
Encoder and resolver based systems are complex systems resulting in high cost components, requiring a larger number of connections, and adding to the overall length and weight of the motor. Accordingly, there is a need in the art for a lower cost, less-complicated motor feedback system that provides both the commutation signals required for controlling the motor and high resolution signals necessary for position/speed control.
In many applications, linear actuators are necessary or desired to provide a reciprocating motion through an actuator member as that member is moved through an actuation stroke. Examples of known linear actuators are U.S. Pat. Nos. 5,491,372 and 5,557,154. Actuators oftentimes do not require the precision or cost of prior art high resolution encoders and resolvers. This is due to, among other factors, feed to force control, the relatively large reduction ratio (e.g., 10:1 or 4:1), and/or the relatively simple point to point motion of an actuator. In the case of motion, this may be contrasted with synchronization of multiple axes in many servo motor systems.
Therefore, a need exists for a linear actuator apparatus and system which offers position/speed control and which addresses the foregoing shortcomings.